Method of making bonded or sintered permanent magnets

ABSTRACT

An isotropic permanent magnet is made by mixing a thermally responsive, low viscosity binder and atomized rare earth-transition metal (e.g., iron) alloy powder having a carbon-bearing (e.g., graphite) layer thereon that facilitates wetting and bonding of the powder particles by the binder. Prior to mixing with the binder, the atomized alloy powder may be sized or classified to provide a particular particle size fraction having a grain size within a given relatively narrow range. A selected particle size fraction is mixed with the binder and the mixture is molded to a desired complex magnet shape. A molded isotropic permanent magnet is thereby formed. A sintered isotropic permanent magnet can be formed by removing the binder from the molded mixture and thereafter sintering to full density.

CONTRACTUAL ORIGIN OF REFERENCE AND GRANT REFERENCE

The United States Government has rights in this invention pursuant tothe Contract No. W-7405-ENG-82 between the U.S. Department of Energy andIowa State University, Ames, Iowa, which contract grants to Iowa StateUniversity Research Foundation, Inc. the right to apply for this patent.The research leading to the invention was supported in part by U.S.Department of Commerce Grant ITA 87-02.

FIELD OF THE INVENTION

The present invention relates to binder-assisted fabrication ofpermanent isotropic magnets and, more particularly, to a method ofmaking permanent isotropic magnets by heat molding mixtures of a binderand an atomized rare earth-transition metal alloy powder and to magnetsthereby produced.

BACKGROUND OF THE INVENTION

A large amount of technological interest has been focused on rareearth-iron-boron alloys (e.g., 26.7 weight % Nd-72.3 weight % Fe-1.0weight % B) as a result of their promising magnetic properties forpermanent magnet applications attributable to the magnetically hard Nd₂Fe₁₄ B phase. Commercial permanent magnets of these alloys havinganisotropic, aligned structure exhibit high potential energy products(i.e., BHmax) of 40-48 MGOe while those having anisotropic, non-alignedstructure exhibit potential energy products of 5-10 MGOe. Such energyproduct levels are much higher than those exhibited by Sm--Co alloys(e.g., SmCo₅ and Sm₂ Co₁₇) previously regarded as having optimummagnetic properties. The rare earth-iron-boron alloys are alsoadvantageous over the SmCo alloys in that the rare earth (e.g., Nd) andFe are much more abundant and economical than Sm and Co. As a result,rare earth-iron-boron permanent magnets are used in a wide variety ofapplications including, but not limited to, audio loud speakers,electric motors, generators, meters, scientific instruments and thelike.

Several distinct processes have been disclosed to fabricate fully dense,permanent magnets from Nd--Fe--B alloys. One process involves forming arapidly solidified, nearly amorphorous ribbon, mechanically comminutingthe ribbon to form flake particulates and then hot pressing and aligningthe flake particulates at elevated temperature in a die cavity. Anotherprocess involves grinding the Nd--Fe--B alloy into fine powder, aligningthe powder in a magnetic field during cold pressing, and sintering thecold pressed powder to near full density. These processes have beenemployed to make aligned (i.e., anisotropic) permanent magnets.

Resin bonding of rapidly solidified ribbon of Nd--Fe--B alloys has beenproposed by R. W. Lee in an article entitled "Hot-pressedNeodymium-iron-boron Magnets", Appl. Phys. Lett. 46: pp. 790-791 (1985)as a technique for fabricating isotropic permanent magnets. In order tomake resin bonded magnets from rapidly solidified, melt-spun ribbon, itis necessary to comminute the friable ribbon into flake particulates andthen to compact the particulates under pressure to a desired shape ofsimple geometry in a compression molding die. The voids of the compactare typically filled with a liquid polymer, such as epoxy and the like,to form a bonded magnet.

It is an object of the present invention to provide a method of makingisotropic permanent magnets from rare earth-transition metal alloysusing a unique alloy powder/binder feedstock blend or mixture thatfacilitates molding of the mixture at relatively low temperatures topreviously unachievable or difficult-to-achieve complex shapes.

It is another object of the present invention to provide a method ofmaking isotropic permanent magnets from rare earth-transition metalalloys wherein low viscosity binder-assisted molding permits relativelylow temperature molding of the feedstock blend or mixture having optimumvolume loading of atomized alloy powder for a particular application.

It is still another object of the present invention to provide isotropicpermanent magnets molded from the alloy powder/binder feedstock blend ormixture.

SUMMARY OF THE INVENTION

The present invention involves a method of making isotropic permanentmagnets by mixing a thermally responsive, low viscosity binder and rareearth-transition metal alloy powder particles which have acarbon-bearing layer thereon that facilitates wetting of the powderparticles by the binder. The mixture is then molded to a threedimensional shape.

In one embodiment of the invention, the powder particulates are formedby atomizing a melt of rare earth-transition metal alloy to formgenerally spherical, rapidly solidified alloy particles. The atomizedparticles are contacted with a carbonaceous material to form thecarbon-bearing layer (typically graphite) in-situ thereon in theatomizing apparatus. The powder particulates are typically sizeclassified into one or more particle size fractions (or classes) suchthat the particles of each size fraction exhibit a grain size in a givenrange and thus generally uniform isotropic magnetic properties. Themixture of sized rare earth-transition metal alloy particulates and thebinder are molded, preferably injection molded, to complex threedimensional shapes.

The binder is selected from a variety of polymeric materials which arethermoplastic or thermosetting and which exhibit low viscosity and otherrheological properties under the molding conditions employed to form themagnet shape so as to readily wet and adhere to the carbon-bearing layerpresent on the alloy powder particles. A preferred binder comprises ablend or mixture of a high melt flow binder (e.g., short chain lowmolecular weight polyethylene) with a stronger, moderate melt flowbinder (clarity low molecular weight polyethylene) in suitableproportions such as, for example, a 2-to-1 mixture by volume.

The binder/alloy powder mixture provides a low viscosity feedstock thatis heat molded to a desired complex magnet shape. Preferably, thefeedstock mixture is molded at relatively low temperature correspondingto the melting temperature of the lowest melting point binder. Othermolding techniques, such as blow molding, extrusion, transfer molding,rotational molding, compression molding, stamping and other lowtemperature/viscosity processes can be employed in practicing theinvention.

The presence of the carbon-bearing layer on the atomized alloy powderimproves wetting and bonding of the alloy powder by the low viscositybinder in the aforementioned molding processes. Moreover, use of fine,spherical alloy powder produced by the atomization process permits highvolume loading of the magnetic alloy powder in the binder, if desired,to provide improved magnetic properties.

Permanent magnets in accordance with the invention are produced asbonded isotropic magnets or, alternately, as sintered, binderlessisotropic magnets. In particular, the bonded magnets of the inventionretain the binder as a matrix for the alloy powder. On the other hand,manufacture of sintered magnets in accordance with the inventioninvolves removing the binder after the molding operation and thensintering to near full density.

The method of the invention can be used to economically produceisotropic permanent magnets of desired microstructure and therebydesired magnetic properties by appropriate selection of (a) the initialparticle size fraction of the atomized alloy powder, (b) the volumeloading of the magnetic alloy powder in the binder, and (c) optionalpost-molding treatments such as binder removal/sintering to which themolded shape may be subjected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet illustrating the sequential method steps of oneembodiment of the invention.

FIG. 2 is a schematic view of apparatus for practicing one embodiment ofthe invention.

FIG. 3 is photomicrograph at 800X of a batch of rapidly solidifiedpowder particles classified into a size fraction of less than 15microns.

FIGS. 4A, 4B are photomicrographs at 1000X of a section of a bondedisotropic permanent magnet made in accordance with Example 1 andexhibiting a homogeneous microstructure and isotropic magneticproperties. FIG. 4A is etched with Nital while FIG. 4B is unetched.

FIG. 5 is a photomicrograph at 400X of a section of a sintered,binderless isotropic permanent magnet made in accordance with Example 2and exhibiting a homogeneous microstructure and isotropic magneticproperties.

FIG. 6 is a bar graph illustrating the distribution in weight % ofparticles as a function of particle size (diameter).

FIG. 7 is a bar graph illustrating the magnetic properties ofas-atomized Nd--Fe--B alloy particles as a function of particle size.

FIG. 8 is a similar bar graph for Nd--Fe--B--La alloy particles.

FIG. 9 is a bar graph for Nd--Fe--B alloy particles illustratingparticle grain size as a function of particle size.

FIG. 10 is a side elevation of a modified atomizing nozzle used in theExamples.

FIG. 11 is a sectional view of the modified atomizing nozzle along lines11--11.

FIG. 12 is a view of the modified atomizing nozzle showing gas jetdischarge orifices aligned with the nozzle tube surface.

FIG. 13 is a bottom plan view of the modified atomizing nozzle.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the various steps involved in practicing oneparticular embodiment of the method of the invention are illustrated. Inthis particular embodiment of the invention, a melt of the appropriaterare earth-transition metal alloy is atomized by a high pressure inertgas process of the type described in copending, commonly assigned U.S.patent application Ser. No. 594,088, now abandoned entitled"Environmentally Stable Reactive Alloy Powders And Method Of MakingSame", to produce fine, environmentally stable, generally spherical,rapidly solidified powder particles of the rare earth-transition metalalloy. The rapid solidification rate that is achieved during his inertgas atomization process is similar to that achieved in melt spinning inso far as there is a beneficial reduction in alloy constituentsegregation during freezing, particularly as compared to the coarsesegregation patterns evident in chill cast ingots.

Referring to FIG. 2, a gas atomization apparatus is shown for atomizingthe melt in accordance with the aforementioned high pressure inert gasatomization process. The apparatus includes a melting chamber 10, a droptube 12 beneath the melting chamber, a powder separator/collectionchamber 14 and a gas exhaust cleaning system 16. The melting chamber 10includes an induction melting furnace 18 and a vertically movablestopper rod 20 for controlling flow of melt from the furnace 18 to amelt atomizing nozzle 22 disposed between the furnace and the drop tube.The atomizing nozzle 22 preferably is of the general supersonic inertgas type described in the Ayers and Anderson U.S. Pat. No. 4,619,845,the teachings of which are incorporated herein by reference, as modifiedin the manner described in Example 1. The atomizing nozzle 22 issupplied with an inert atomizing gas (e.g., argon, helium) from asuitable source 24, such as a conventional bottle or cylinder of theappropriate gas. As shown in FIG. 2, the atomizing nozzle 22 atomizesthe melt in the form of a spray of generally spherical, molten dropletsD discharged into the drop tube 12.

Both the melting chamber 10 and the drop tube 12 are connected to anevacuation device (e.g., vacuum pump) 30 via suitable ports 32 andconduits 33. Prior to melting and atomization of the melt, the meltingchamber 10 and the drop tube 12 are evacuated to a level of 10⁻⁴atmosphere to substantially remove ambient air. Then, the evacuationsystem is isolated from the chamber 10 and the drop tube 12 via thevalves 34 shown and the chamber 10 and drop tube 12 are positivelypressurized by an inert gas (e.g., argon to about 1.1 atmosphere) toprevent entry of ambient air thereafter.

The drop tube 12 includes a vertical drop tube section 12a and a lateralsection 12b that communicates with the powder collection chamber 14. Thedrop tube vertical section 12a has a generally circular cross-sectionhaving a diameter in the range of 1 to 3 feet, a diameter of 1 footbeing used in the Examples set forth below. As will be explained below,the diameter of the drop tube section 12a and the diameter of thesupplemental reactive gas jet 40 are selected in relation to one anotherto provide a reactive gas zone or halo H extending substantially acrossthe cross-section of the drop tube vertical section 12a at the zone H.

The length of the vertical drop tube section 12a is typically about 9 toabout 16 feet, a preferred length being 9 feet being used in theExamples set forth below, although other lengths can be used inpracticing the invention. A plurality of temperature sensing means 42(shown schematically), such as radiometers or laser doppler velocimetrydevices, may be spaced axially apart along the length of the verticaldrop section 12a to measure the temperature of the atomized droplets Das they fall through the drop tube and cool in temperature.

The supplemental reactive gas jet 40 referred to above is disposed atlocation along the length of the vertical drop section 12a where thefalling atomized droplets D have cooled to a reduced temperature(compared to the droplet melting temperature) at which the droplets haveat least a solidified exterior surface thereon and at which the reactivegas in the zone H can react with one or more reactive alloying elementsof the shell to form a protective barrier layer (reaction product layercomprising a refractory compound of the reactive alloying element) onthe droplets whose depth of penetration into the droplets iscontrollably limited by the presence of the solidified surface as willbe described below.

In particular, the jet 40 is supplied with reactive gas (e.g., nitrogen)from a suitable source 41, such as a conventional bottle or cylinder ofappropriate gas, through a valve and discharges the reactive gas in adownward direction into the drop tube to establish the zone or halo H ofreactive gas through which the droplets travel and come in contact forreaction in-situ therewith as they fall through the drop tube. Thereactive gas is preferably discharged downwardly in the drop tube tominimize gas updrift in the drop tube 12. The flow patterns establishedin the drop tube by the atomization and falling of the dropletsinherently oppose updrift of the reactive gas. As a result, a reactivegas zone or halo H having a more or less distinct upper boundary B andless distinct lower boundary extending to the collection chamber 14 isestablished in the drop tube section 12a downstream from the atomizingnozzle in FIG. 2. As mentioned above, the diameter of the drop tubesection 12a and the jet 40 are selected in relation to one another toestablish a reactive gas zone or halo that extends laterally across theentire drop tube cross-section. This places the zone H in the path ofthe falling droplets D so that substantially all of the droplets traveltherethrough and contact the reactive gas.

The temperature of the droplets D as they reach the reactive gas zone Hwill be low enough to form at least a solidified exterior surfacethereon and yet sufficiently high as to effect the desired reactionbetween the reactive gas and the reactive alloying element(s) of thedroplet composition. The particular temperature at which the dropletshave at least a solidified exterior shell will depend on the particularmelt composition, the initial melt superheat temperature, the coolingrate in the drop tube, and the size of the droplets as well as otherfactors such as the "cleanliness" of the droplets; i.e., theconcentration and potency of heterogeneous catalysts for dropletsolidification.

Preferably, the temperature of the droplets when they reach the reactivegas zone H will be low enough to form at least a solidified exteriorskin or shell of a detectable, finite shell thickness; e.g., a shellthickness of at least about 0.5 micron. Even more preferably, thedroplets are solidified from the exterior surface substantially to thedroplet core (i.e., substantially through their diametral cross-section)when they reach the reactive gas zone H. As mentioned above, radiometersor laser doppler velocimetry devices, may be spaced axially apart alongthe length of the vertical drop section 12a to measure the temperatureof the atomized droplets D as they fall through the drop tube and coolin temperature, thereby sensing or detecting when at least a solidifiedexterior shell of finite thickness has formed on the droplets. Theformation of a finite solid shell on the droplets can also be readilydetermined using a physical sampling technique in conjunction withmacroscopic and microscopic examination of the powder samples taken atdifferent axial locations downstream from the atomizing nozzle in thedrop tube 12. This technique is disclosed in aforementioned copendingU.S. patent application Ser. No. 594,088, now abandoned, the teachingsof which are incorporated herein by reference.

Referring to FIG. 2, prior to atomization, a thermally decomposableorganic material is deposited on a splash member 12c disposed at thejunction of the drop tube vertical section 12a and lateral section 12bto provide sufficient gaseous carbonaceous material in the drop tubesections 12a,12b below zone H as to form a carbon-bearing (e.g.,graphite) layer on the hot droplets D after they pass through thereactive gas zone H. The organic material may comprise an organic cementto hold the splash member 12c in place in the drop tube 12. Alternately,the organic material may simply be deposited on the upper surface orlower surface of the splash member 12c. In any event, the material isheated during atomization to thermally decompose it and release gaseouscarbonaceous material into the drop tube sections 12a,12b below zone H.An exemplary organic material for use comprises Duco® model cement thatis applied in a uniform, close pattern to the bottom of the splashmember 12c to fasten it to the elbow 12b. Also, the Duco cement isapplied as a heavy bead along the exposed uppermost edge of the splashmember 12c after the initial fastening to the elbow. The Duco organiccement is subjected during atomization to temperatures in excess of 500°C. so that the cement is thermally decomposed and acts as a source ofgaseous carbonaceous material to be released into the drop tube sections12a,12b beneath the zone H. The extent of heating and thermaldecomposition of the cement and, hence, the concentration ofcarbonaceous gas available for powder coating is controlled by theposition of the splash member 12c, particularly the exposed uppermostedge, relative to the initial melt splash impact region and the centralzone of the spray pattern. To maximize the extent of heating and thermaldecomposition, additional Duco cement can be laid down (deposited) asstripes on the upper surface of the splash member 12c.

Alternately, a second supplemental jet 50 is shown disposed downstreamof the first supplemental reactive gas jet 40. The second jet 50 isprovided to receive a carbonaceous material, such as methane, argonlaced with paraffin oil and the like, from a suitable source (not shown)for discharge into the drop tube section 12a to form the carbonaceous(e.g., graphitic carbon) coating or layer on the hot droplets D afterthey pass through the reactive gas zone H.

Powder collection is accomplished by separation of the powderparticles/gas exhaust stream in the tornado centrifugal dustseparator/collection chamber 14 and by retention of separated powderparticles in the valved particle-receiving container, FIG. 2.

In practicing the present invention using the apparatus of FIG. 2, themelt may comprise various rare earth-transition metal alloys selected toachieve desired isotropic magnetic properties. The rare earth-transitionmetal alloys typically include, but are not limited to, Tb--Ni, Tb--Feand other refrigerant magnetic alloys and rare earth-iron-boron alloysdescribed in U.S. Pat. Nos. 4,402,770; 4,533,408; 4,597,938 and4,802,931, the teachings of which are incorporated herein by reference,where the rare earth is selected from one or more of Nd, Pr, La, Tb, Dy,Sm, Ho, Ce, Eu, Gd, Er, Tm, Yb, Lu, Y, and Sc. The lower weightlanthanides (Nd, Pr, La, Sm, Ce, Y, Sc) are preferred. Rareearth-iron-boron alloys, especially Nd--Fe--B alloys comprising about 26to 36 weight % Nd, about 62 to 68 weight % Fe and 0.8 to 1.6 weight % B,are preferred in practicing the invention as a result of theirdemonstrated excellent magnetic properties.

Rare earth-iron-boron alloys rich in rare earth (e.g., at least 27weight %) and rich in boron (e.g., at least 1.1 weight %) are preferredto promote formation of the hard magnetic phase, Nd₂ Fe₁₄ B, in anequiaxed, blocky microstructure, and minimize, preferably avoid,formation of the ferritic Fe phase in all particle sizes produced. TheNd--Fe--B alloys rich in Nd and B were found to be substantially free ofprimary ferritic Fe phase, which was observed in some particle sizes(e.g., 10 to 20 microns) for Fe rich and near-stoichiometric alloycompositions. Alloyants such as Co, Ga, La, and others may be includedin the alloy composition, such as 31.5 weight % Nd- 65.5 weight % Fe-1.408 weight % B- 1.592 weight % La and 32.6 weight % Nd- 50.94 weight %Fe- 14.1 weight % Co- 1.22 weight % B- 1.05 weight % Ga.

In the case of the rare earth-transition metal-boron alloys, the rareearth and boron are reactive alloying elements that must be maintainedat prescribed concentrations to provide desired magnetic properties inthe powder product.

The reactive gas may comprise a nitrogen bearing gas, oxygen bearinggas, carbon bearing gas and the like that will form a stable reactionproduct comprising a refractory compound, particularly anenvironmentally protective barrier layer, with the reactive alloyingelement of the melt composition. Illustrative of stable refractoryreaction products are nitrides, oxides, carbides, borides and the like.The particular reaction product formed will depend on the composition ofthe melt, the reactive gas composition as well as the reactionconditions existing at the reactive gas zone H. The protective barrier(reaction product) layer is selected to provide protection againstenvironmental constituents, such as air and water in the vapor or liquidform, to which the powder product will be exposed during subsequentfabrication to an end-use shape and during use in the intended serviceapplication.

The depth of penetration of the reaction product layer into the dropletsis controllably limited by the droplet temperature (extent of exteriorshell solidification) and by the reaction conditions established at thereactive gas zone H. In particular, the penetration of the reactionproduct layer (i.e., the reactive gas species, for example, nitrogen)into the droplets is limited by the presence of the solidified exteriorshell so as to avoid selective removal of the reactive alloying element(by excess reaction therewith) from the droplet core composition to aharmful level (i.e., outside the preselected final end-use concentrationlimits) that could substantially degrade the end-use properties of thepowder product. For example, with respect to the rare earth-transitionmetal-boron alloys, the penetration of the reaction product layer islimited to avoid selectively removing the rare earth and the boronalloyants from the droplet core composition to a harmful level (outsidethe prescribed final end-use concentrations therefor) that wouldsubstantially degrade the magnetic properties of the powder product inmagnet applications. In accordance with the invention, the thickness ofthe reaction product layer formed on rare earth-transition metal-boronalloy powder is limited so as not to exceed about 500 angstroms,preferably being in the range of about 200 to about 300 angstroms, forpowder particle sizes in the range of about 1 to about 75 microns,regardless of the type of reaction product layer formed. Generally, thethickness of the reaction product layer does not exceed 5% of the majorcoated powder particle dimension (i.e., the particle diameter) to thisend.

The reaction barrier (reaction product) layer may comprise multiplelayers of different composition, such as an inner nitride layer formedon the droplet core and an outer oxide type layer formed on the innerlayer. The types of reaction product layers formed again will dependupon the melt composition and the reaction conditions present at thereactive gas zone H.

As mentioned above, a carbon-bearing (graphitic carbon) layer is formedin-situ on the reaction product layer by various techniques. Such agraphitic carbon layer is formed to a thickness of at least about 1monolayer (2.5 angstroms) regardless of the technique employed. Thelayer provides protection to the powder product against suchenvironmental constituents as liquid water or water vapor as, forexample, is present in humid air. Importantly, the layer alsofacilitates wetting of the powder product by polymer binders, such aspolyolefins (e.g., polyethylenes) as described below in injectionmolding of the binder/alloy powder mixtures to form complex, end-usemagnet shapes.

The invention is not limited to the particular high pressure inert gasatomization process described in the patent and may be practiced usingother atomization nozzles, such as annular slit, close-coupled nozzlesor conventional free-fall nozzles that yield rapidly solidified powderhaving appropriate sizes for use in the fabrication of isotropicpermanent magnets.

Referring to FIG. 1, one embodiment of the invention involves producingenvironmentally stable, generally spherical, rapidly solidified powderparticles using the high pressure inert gas atomizationprocess/apparatus described in Example 1 such that the rareearth-transition metal alloy particles fall within a given particle size(diameter) range (and thus within a given grain size range) wherein themajority of the particles exhibit particle diameters less than a givendiameter determined to exhibit desirable magnetic properties for theparticular alloy composition and magnet service application involved.For example, in practicing the invention to make Nd--Fe--B alloymagnets, the powder particles produced using the high pressure inert gasatomization process/apparatus typically fall within a particle size(diameter) range of about 1 micron to about 100 microns with a majority(e.g., 66-68% by weight) of the particles having a diameter less thanabout 44 microns, typically from about 3 to about 44 microns.Preferably, a majority of the particles are less than about 38 micronsin diameter, a particle size found to yield optimum magnetic propertiesin the as-atomized condition as will become apparent below. FIG. 5illustrates in bar graph form a typical distribution in weight % of twobatches of Nd--Fe--B--La alloy particles as a function of particle size.The composition (in weight %) of the alloys before atomization is setforth below in the Table:

                  TABLE                                                           ______________________________________                                                    Nd   Fe        B      La                                          ______________________________________                                        Alloy BT-1-190                                                                              31.51  65.49     1.32 1.597                                     Alloy BT-1-216                                                                              33.07  63.93     1.32 1.68                                      ______________________________________                                    

Both alloys BT-1-190 and BT-1-216 were atomized under conditions similarto those set forth in Example 1. With Nd--Fe--B type alloys, the Ndcontent of the alloy was observed to be decreased by about 1-2 weight %in the atomized powder compared to the melt as a result of melting andatomization, probably due to reaction of the Nd during melting withresidual oxygen and formation of a moderate slag layer on the meltsurface. The iron content of the powder increased relatively as a resultwhile the B content remained generally the same. The initial meltcomposition can be adjusted to accommodate these effects.

FIG. 5 reveals that a majority of the as-atomized powder particles fallin the particle size (diameter) range of less than 45 microns, even moreparticularly less than 38 microns (i.e., -38 on the abscissa). Inparticular, greater than 60% (about 66-68%) by weight of the particlesexhibit particle diameter of less than 38 microns found to exhibitoptimum magnetic properties in the as-atomized condition as will becomeapparent. These weight distributions were determined by hand sifting(screening) an entire batch of powder through a full range of ASTM wovenwire screens.

The advantage of producing the alloy powder particles in the mannerdescribed above is evident in FIGS. 6 and 7. In FIGS. 6 and 7, themagnetic properties (namely, coercivity, remanence and saturation) ofas-atomized powder as a function of particle size is set forth for alloyBT-1-162 (32.5 weight % Nd-66.2 weight % Fe-1.32 weight % B, FIG. 6) andthe aforementioned alloy BT-1-190 (FIG. 7). The alloys were atomizedunder like conditions similar to those set forth in Example 1. TheFigures demonstrate that coercivity and, to a lesser extent, remanenceappear to vary as a function of particle size in both alloys. Elevatedlevels of coercivity and remanence are observed in both alloys asparticle size (diameter) is reduced below about 38 microns. On the otherhand, saturation magnetization of both alloys remains relativelyconstant over the range of particle sizes. For alloy BT-1-162, thecoercivity falls significantly as particle size is reduced below about 5microns. These results correlate with grain size measurements whichreveal a continuous decrease in grain size with reduced particle size;e.g., from a grain size of about 500 nm for 15-38 micron particles toabout 40-70 nm for less than 5 micron particles; for example, as shownin FIG. 8 for alloy BT-1-162. Magnetic property differences betweenpowder size classes were due to differences in the microcrystallinegrain size within each particle.

From FIGS. 6 and 7, it is apparent that the magnetic properties,particularly the coercivity, of the alloy powder increase with decreasedparticle size to a maximum of about 10-11 kOe for powder particles ofabout 15-38 microns diameter, and then decrease for particles of furtherreduced size. Moreover, it is apparent that near optimum overallmagnetic properties are exhibited by the as-atomized alloy particles inthe general particle size (diameter) range of about 3 microns to about44 microns and, more particularly, about 5 to about 40 microns where themajority of the particles are produced by the high pressure inert gasatomization process described above. Thus, the yield of as-atomizedpowder particles possessing useful magnetic properties is significantlyenhanced in practicing the invention as described above.

Typically, in the above-described embodiment of the invention, eachbatch of alloy particles produced using the high pressure inert gasatomization process of Example 1 is initially size classified by, forexample, sifting (screening) through an ASTM 44 micron woven wire meshscreen This preliminary size classifying operation substantially removesparticles greater than 44 microns diameter from the batch and therebyincreases the percentage of finer particles in each batch. Thispreliminary screening operation is conducted in a controlled atmosphere(nitrogen) glove box after the contents of the sealed powder container,FIG. 2, are opened in the glove box.

Referring again to FIG. 1, in another embodiment of the invention, therapidly solidified powder produced by the high pressure inert gasatomization process is subjected to the preliminary size classifying(screening) operation described above and also to one or more additionalsize classifying operations to form one or more particle size fractionsor classes wherein each fraction or class comprises powder particleshaving a particle size (diameter) in a given relatively narrow range.For example, for a typical batch of high pressure inert gas atomizedNd--Fe--B powder (e.g., BT-1-162 described above), the followingparticle size fractions or classes having the listed range of particlesizes (diameters) are provided by carrying out an air classifyingoperations on the batch using an air classifying procedure to bedescribed:

Fraction #1- about 38 to about 15 microns (diameter)

Fraction #2- about 15 to about 10 microns (diameter)

Fraction #3- about 10 to about 5 microns (diameter)

Fraction #4- about 5 to about 3 microns (diameter)

In particular, the rapidly solidified powder particles were airclassified using a commercially available air classifier sold as modelA-12 under the name Majac Acucut air classifier by Hosokawa MiconInternational Inc., 10 Chantham Rd., Summit, N.J. In producing theparticle size fractions #1, #2, #3 and #4 described above, the rapidlysolidified powder was air classified using a blower pressure of 135inches water, an ejector pressure of 50 psi with rotor speeds of 507rpm, 715 rpm, 1145 rpm and 1700 rpm to yield the particle size fractions#1, #2, #3 and #4, respectively.

As is apparent, in any given particle size fraction or class, the powderparticles fall within a given narrow range of mean particle sizes(diameters). As a result, the powder particles in each particle sizefraction or class exhibit a rapidly solidified microstructure,especially grain size, also within a very narrow range. In this way, theclassifying operation is effective to provide isotropic magnetic articleproperties. For example the following grain size ranges were observedfor each particle size fraction:

Fraction #1- about 490 nm to about 500 nm grain size

Fraction #2- about 210 nm to about 220 nm grain size

Fraction #3- about 115 nm to about 130 nm grain size

Fraction #4- about 60 nm to about 75 nm grain size

A plurality of particle size (air) fractions or classes having quiteuniform particle microstructures (grain sizes) within each fraction orclass are thereby provided by the size classifying operation depicted inFIG. 1. Depending upon the particular magnetic properties desired in themagnet, a particular particle size fraction or class having theappropriate microstructure can then be selected to this end for furtherprocessing in accordance with the invention to produce the desiredmagnet. A different particle size fraction or class can be chosen forfurther processing in accordance with the invention in the eventslightly different magnetic/mechanical properties are specified by themagnet user or manufacturer.

Referring to FIG. 1, the alloy powder particles, either as initiallysize classified (screened) in accordance with the first embodiment ofthe invention, as air classified in accordance with the secondembodiment of the invention or as-atomized, are then mixed or blendedwith a thermally responsive, low viscosity binder, such as athermoplastic or thermosetting polymeric binder, to provide a feedstockthat can be formed (molded) to desired shape under relatively low heatand pressure (e.g., injection molding conditions). The binder and thealloy powder are mixed in proportions dependent upon the alloy powderemployed, the binder employed as well as the desired volume loading ofmagnetic powder particles in the feedstock. High volume loadings ofpowder in the binder are achievable as a result of the fine, sphericalpowder particles produced by the high pressure inert gas atomizationprocess. For example, powder volume loadings of about 75 to about 80volume % are possible in practicing the invention. However, theinvention is not so limited and may be practiced to make powder-filledpolymers having less than 50 volume % powder therein depending on themagnet properties desired. Blends of particles of different sizes can beused to achieve optimal volume loading.

The low viscosity binder may be selected from certain materials whichare effective to wet and bond the outer, carbon-bearing layer on thepowder particles under the particular molding conditions involved.Binders useful in practicing the present invention are generallycharacterized as having low viscosity (e.g., 100 to 10 Pas for aspecified shear rate of 50 to 500 mm per mm per second). The binder mayinclude a coupling agent, such as glycerol, titanate, stearic acid,polyethylene glycol, polyethylene oxide, humic acid, ethoxylated fattyacid and other known coupling/processing aid agents to achieve higherloading of powder in the binder. Binders exhibiting such propertiesinclude 66 weight % PE#1 (Grade 6 polyethylene homopolymer sold byAllied Corp., Morristown, N.J.) and 33 weight % PE#2 (Clarity linear lowdensity polyethylene Grade 5272 - See ASTM NA153 or, alternately PE#2may comprise PE2030 (#38645) available from CFC Prime Alliance, DesMoines, Iowa), 64 weight % PE#1 - 30 weight % PE#2 - 5 weight % stearicacid (Grade A-292 sold by Fisher Scientific Co.), 75 weight % PE#1 - 25weight % PE#2, 72 weight % PE#1 - 23 weight % PE#2 - 5 weight % stearicacid, 44 volume % corn oil - 54 weight % polystyrene - 4.7 volume %stearic acid, 65 weight % PE#1 - 32 weight % PE#2 - 2 weight % LICA-12(a titanate available from Kenrich Petrochemcial Corp.), and polystyrene(1.045 gm/cc available from Huntsman Chemical Company, Salt Lake City,Utah). A Teflon® (Grade 7A available from DuPont) binder is useful forcompression molding.

A preferred low viscosity binder for use in the invention comprises amixture of a high melt flow, short chain low molecular weightpolyethylene (e.g., PE#1 - melting point of 106° C.) and a stronger,moderate melt flow, low molecular weight polyethylene (e.g., PE#2 -softening point of about 130° C.) preferably in a 2-to-1 volume % ratio,as set forth in the Examples.

The binder and the alloy powder are typically mixed or blended bymoderate to high shear mixing to provide a homogeneous, low viscosityfeedstock. The feedstock viscosity typically is selected in the range ofabout 10 to about 100 Pas for the injection molding process described inthe Examples set forth hereinbelow. Of course, the particular viscositylevel used will depend on the particular binder employed, the powderemployed and powder volume loading employed as well as the type ofmolding process employed.

Molding of the low viscosity feedstock is typically effected byinjection molding using equipment currently employed in the plasticindustry to injection mold metal-filled polymers; e.g., as described inby R. M. German, Powder Injection Molding, Metals Powder IndustryFederation, Princeton, N.J. 1990, the teachings of which areincorporated herein by reference. Highly complex three dimensionalshapes can be formed by injection molding into a suitable die or moldingcavity. However, the invention is not limited to such injection moldingprocesses and may be practiced using blow molding, extrusion,co-extrusion, transfer molding, rotational molding, compression molding,stamping and other low viscosity forming processes.

Injection molding is typically conducted under relatively lowtemperature and pressure conditions such as, for example, a temperatureof about 25° to about 170° C. and injection pressures of about 50 toabout 3000 psi. The molding temperature is selected to melt the lowestmelting point binder constituent (e.g., PE#1 described above) whilesoftening the other binder constituent (e.g., PE#2 described above). Ofcourse, the molding parameters employed will depend upon the particularmolding process used as well as the binder and powder types and volumeloading used. Higher pressures are needed for more complex mold cavitygeometry and runner and gating systems. Molding time will also varydepending on these same factors. Once the magnet compact is molded toshape, it is cooled to 25° to 50° C. and removed from the molding diewhereupon the binder maintains the molded shape.

After the molding operation, the magnet compact may be used as a bondedmagnet with minimal finishing operations such as coating the magnet withteflon for environmental protection purposes. For bonded magnets, theas-molded compact will correspond closely in shape to the desired magnetconfiguration for the intended service application so that little or nomachining is required. Alternately, the binder may be removed from themolded compact by a controlled thermal cycle or chemical cycle and thenthe binderless compact is sintered to near full density. If the bindercomprises the 2 to 1 mixture of PE#1 and PE#2 described hereinabove, thebinder can be removed by heating to 550° C. in a protective atmosphere,such as argon or vacuum (10⁻⁶ torr), to protect the magnet alloy powderfrom oxidation, for an appropriate time to burn out the binder. The samebinder can also be removed chemically by solventcondensation-evaporation using heptane at 60° C. as described in "TheEffects of Binder on the Mechanical Properties of Carbonyl IronProducts", K. D. Hens, S. T. Lin, R. M. German and D. Lee, J. of Metals,1989, Vol. 41, No. 8, pp. 17-21, the teachings of which are incorporatedherein by reference. If the binder is thusly removed, the compact willundergo some shrinkage which must be taken into consideration indimensioning the injection molding die so that the desired size ofsintered magnet is ultimately produced.

Bonded magnets made in accordance with the invention typically exhibitenergy products (BHmax) of about 3 to about 6 MGOe. Sintered magnets ofthe invention typically exhibit energy products of about 5 to about 8MGOe.

The following Examples are offered to illustrate, but not limit, theinvention.

EXAMPLE 1

The melting furnace of FIG. 2 was charged with an Nd-16 weight % Femaster alloy as-prepared by thermite reduction, an Fe--B alloycarbo-thermic processed and available from Shieldalloy MetallurgicalCorp., and electrolytic Fe obtained from Glidden Co. The quantity ofeach charge constituent was controlled to provide a melt composition ofabout 33.0 weight % Nd- 65.9 weight % Fe- 1.1 weight % B. The charge wasmelted in the induction melting furnace after the melting chamber andthe drop tube were evacuated to 10⁻⁴ atmosphere and then pressurizedwith argon to 1.1 atmospheres. The melt was heated to a temperature of1650° C. After a hold period of 10 minutes to reduce (vaporize) Capresent in the melt (from the thermite reduced Nd--Fe master alloy) tomelt levels of 50-60 ppm by weight, the melt was fed to the atomizingnozzle by gravity flow upon raising of the stopper rod. The atomizingnozzle 22 was of the type described in U.S. Pat. No. 4,619,845 asmodified (see FIGS. 10-13) to include (a) a divergent manifold expansionregion 120 between the gas inlet 116 and the arcuate manifold segment118 and (b) an increased number (i.e., 20) of gas jet discharge orifices130 that are NC machined to be in close tolerance tangency T (e.g.,within 0.002 inch, preferably 0.001 inch) to the inner bore 133 of thenozzle body 104 to provide improved laminar gas flow over thefrusto-conical surface 134 of the two-piece nozzle melt supply tube 132(i.e., inner boron nitride melt supply tube 132c and outer Type 304stainless steel tube 132b with thermal insulating space 132dtherebetween). The divergent expansion region 120 minimizes wallreflection shock waves as the high pressure gas enters the manifold toavoid formation of standing shock wave patterns in the manifold, therebymaximizing filling of the manifold with gas. The manifold had an r₀ of0.3295 inch, r₁ of 0.455 inch and r₂ of 0.642 inch. The number ofdischarge orifices 130 was increased from 18 (patented nozzle) to 20 butthe diameter thereof was reduced from 0.0310 inch (patented nozzle) to0.0292 inch to maintain the same gas exit area as the patented nozzle.The modified atomizing nozzle was found to be operable at lower inertgas pressure while achieving more uniformity in the particles sizesproduced; e.g., to increase the percentage of particles falling in thedesired particle size range (e.g., less than 38 microns) for optimummagnetic properties for the Nd--Fe--B alloy involved from about 25weight % to about 66-68 weight %. The yield of optimum particle sizeswas increased to improve the efficiency of the atomization process. Themodified atomizing nozzle is described in copending U.S. patentapplication entitled, "Improved Atomizing Nozzle and Process" U.S. Pat.No. 5,125,574, the teachings of which are incorporated herein byreference.

Argon atomizing gas at 1050 psig was supplied to the atomizing nozzle inaccordance with the aforementioned patent. The reactive gas jet waslocated 75 inches downstream of the atomizing nozzle in the drop tube.Ultra high purity (99.95%) nitrogen gas was supplied to the jet at apressure of 100 psig for discharge into the drop tube to establish anitrogen gas reaction zone or halo extending across the drop tube suchthat substantially all the droplets traveled through the zone. At thisdownstream location from the atomizing nozzle, the droplets weredetermined to be at a temperature of approximately 1000° C. or less,where at least a finite thickness solidified exterior shell was presentthereon. After the droplets traveled through the reaction zone, theywere collected in the collection container of the collection chamber(see FIG. 2). The solidified powder product was removed from thecollection chamber when the powder reached approximately 22° C.

The powder particles comprised a core having a particular magneticend-use composition, an inner protective refractory layer and an outercarbonaceous (graphitic carbon) layer thereon. The reaction productlayer formed on the rare earth-transition metal alloy powder is limitedso as not to exceed about 500 A, preferably being in the range of about200 to about 300 angstrom. Auger electron spectroscopy (AES) was used togather surface and near surface chemical composition data on theparticles using in-situ ion milling to produce a depth profile. The AESanalysis indicated an inner surface layer enriched in nitrogen, boronand Nd corresponding to a mixed Nd--B nitride (refractory reactionproduct). The first inner layer was about 150 to about 200 angstroms inthickness. A second inner layer enriched in Nd, Fe, and oxygen wasdetected atop the nitride layer. This second layer corresponded to themixed oxide of Nd and Fe (refractory reaction product) and is believedto have formed as a result of decomposition and oxidation of the initialnitride layer while the powder particles were still at elevatedtemperature. The second layer was about 100 angstroms in thickness. Anoutermost third layer of graphitic carbon was also present on theparticles. This outermost layer was comprised of graphitic carbon withsome traces of oxygen and had a thickness of at least about 3monolayers. This outermost carbon layer is believed to have formed as aresult of thermal decomposition of the Duco® cement (used to hold thesplash member 12c in place) and subsequent deposition of carbon on thehot particles after they passed through reactive gas zone H so as toproduce the graphitic carbon film or layer thereon. Subsequent atomizingruns conducted with and without excess Duco cement present confirmedthat the cement was functioning as a source of gaseous carbonaceousmaterial for forming the graphite layer on the particles. The Ducocement is typically present in an amount of about one (1) ounce foratomization of a 4.5 kilogram melt to produce the graphite coating onthe particles.

The collected powder particles ranged in size from about 1 to about 100microns with a majority of the particles being less than about 38microns in diameter. The powder particles were first screened using ASTM44 micron woven wire mesh and then air classified into a particle sizefraction where the particle diameters were less than 15 microns. Aportion of this high pressure gas atomized powder (HPGA powder) wasmixed with two different binders (see Table 1A) and molded into 3.65inch diameter disks with each disk having two concentric recessed ringsformed therein to a recess depth of 0.15 inch and radii of 1.675 and1.017 inches. This disk geometry was selected as a demonstration of ashape that would be very difficult to make with conventional press andsinter processes. The molding was conducted at 140° C. and injectionpressure of 50 psi in a laboratory scale, plunger type injection moldingapparatus. Table 1A provides a description of the molding results. Thebonded magnet compact produced using the different binders exhibitedmagnetic properties set forth in Table 1B. FIG. 4A, B illustrates themicrostructure of the bonded magnet produced.

                  TABLE 1A                                                        ______________________________________                                        Lab Scale Injectin Molding Using A vertical Plunger Molder                    Mixture          Comments                                                     ______________________________________                                        50 vol. %-PE #1  Powder/binder was mixed well.                                50 vol. %-HPGA Powder                                                                          The as molded 3" disk was too                                (-15 microns)    brittle to be ejected using the                                               pin configuration without flow                                                lines and cracks. No distortion                                               was observed.                                                50 vol. % (66 wt. % PE #1-33                                                                   Polymers were precompounded                                  wt. % PE #2)     and mixed well with powder.                                  50 vol. %-HPGA powder                                                                          The as molded 3" disk had much                               (<15 microns)    more elasticity during shrinkage                                              and ejection from the mold.                                                   Good molding conditions resulted                                              in an undistorted, crack-free disk.                          ______________________________________                                    

                  TABLE 1B                                                        ______________________________________                                        BHmax        Coercivity Remanence Saturation                                  (MGOe)       (kOe)      (kGauss)  (kGauss)                                    ______________________________________                                        Sample 1                                                                              4.6      3.0        5.8     11.0                                      Sample 2                                                                              6.7      7.5        6.3     12.0                                      ______________________________________                                    

EXAMPLE 2

A portion of the air classified powder of Example 1 was mixed with thePE#1/PE#2 binder (66.6 weight % PE#1/33.3 weight % PE#2 ) but in adifferent volumetric proportion relative to the HGPA powder as set forthin Table 2A (i.e., 35 vol. % PE#1/PE#2 binder versus 65 vol. % HPGApowder). The mixture was molded to the aforementioned disk configurationusing the same molding equipment/parameters described above forExample 1. The molded compact was debound (i.e., binder removed) byheating to 550° C. at 1° C./min and then sintered at 800° C. for 1 hourunder an inert atmosphere. The sintered magnet compact exhibitedmagnetic properties set forth in Table 2B. FIG. 5 illustrates themicrostructure of the sintered magnet produced.

                  TABLE 2A                                                        ______________________________________                                        Lab Scale Injection Molding Using A Vertical Plunger Molder                   Mixture        Comments                                                       ______________________________________                                        35 vol. % (66 wt. %                                                                          Polymer and powder blended well.                               PE #1-33 wt. % PE #2)                                                                        However, the mixture was more                                  65 vol. %-HPGA powder                                                                        viscous and was not resistant to                               -15 microns    thermal cracking, cooling and                                                 shrinkage in the mold.                                         ______________________________________                                    

EXAMPLE 3

A batch of powder particles was atomized from a melt comprising 34.7weight % Nd- 63.89 weight % Fe- 1.31 weight % B, screened and airclassified into particle size fraction less than 15 microns similar toExample 1. This particle size fraction was mixed with the PE#1/PE#2binder/mixture set forth in Table 1 in a 50--50 volume percentage basisof the PE#1/PE#2 binder to HPGA powder. The mixture of binder and powderparticles was then injection molded as in Example 1 to the disk geometrydescribed there. Table 3A provides a description of the mold results.The magnetic properties of the bonded magnetic compact are set forth inTable 3B.

                  TABLE 3A                                                        ______________________________________                                        Lab Scale Injection Molding Using A Vertical Plunger Mold                     Mixture                 Comments                                              ______________________________________                                        50 vol. % (66 wt. % PE #1-33 wt. % PE #2)                                                             Powder and                                            50 vol. % HPGA Powder (-15 microns)                                                                   binder blended                                                                well and molded                                                               well                                                  ______________________________________                                    

                  TABLE 3B                                                        ______________________________________                                        BHmax        Coercivity Remanence Saturation                                  (MGOe)       (kOe)      (kGauss)  (kGauss)                                    ______________________________________                                        Sample 3                                                                              2.09     4.5        4.58    8.7                                       ______________________________________                                    

In Examples 1-3, the powder particles were air classified to less than15 microns diameter. Powder particles classified in the size range of15-38 microns in diameter are believed to offer optimum magneticproperties (e.g., as shown in FIGS. 7-8) and thus should provideimproved magnetic properties for bonded/sintered magnet compactsproduced by similar Examples.

EXAMPLE 4

A batch of powder particles was atomized from a melt comprising 31.5weight % Nd- 65.5 weight % Fe- 1.408 weight % B- 1.592 weight % La andclassified into particle size fraction of less than 38 microns to 15microns. This particle size fraction was mixed with Teflon(polytetrafluoroethylene - Grade 7A sold by DuPont, Wilmington, Del.) ina volume proportion of 60 volume % powder to 40 volume % Teflon. Themixture of binder and powder was then compression molded at 180°-220° C.to a 1 inch diameter by 0.25 inch thick disk. The following Table 4 setsforth the magnetic properties.

                  TABLE 4                                                         ______________________________________                                        BHmax    Coercivity   Remanence Saturation                                    (MGOe)   (kOe)        (kGauss)  (kGauss)                                      ______________________________________                                        2.23     6.2          3.75      7.39                                          ______________________________________                                    

EXAMPLE 5

Batches of powder particles were also successfully molded to form 6 inchdiameter by 6 inch long hollow cylinders having a wall thickness of 0.2inch. The first batch was atomized from a melt comprising 33.0 weight %Nd- 65.9 weight % Fe- 1.1 weight % B, and the second batch from a meltcomprising 32.6 weight % Nd- 50.94 weight % Fe- 1.22 weight % B- 14.1weight % Co- 1.05 weight % Ga. Each batch was atomized and classifiedinto particle size fraction of less than 38 microns to 15 microns. Eachparticle size fraction was mixed with Teflon (Grade 7A) in a 60:40volume % ratio of powder to Teflon. The mixture was then rotationalmolded at 170° C. and 800 rpm to successfully form the hollow cylinders.

While the invention has been described in terms of specific embodimentsthereof, it is not intended to be limited thereto but rather only to theextent set forth hereafter in the following claims.

We claim:
 1. A method of making a bonded isotropic permanent magnet,comprising the steps of:a) forming a carbon layer on rareearth-transition metal alloy particles by contacting said alloyparticles and a carbonaceous material, (b) mixing the rareearth-transition metal alloy particles having the carbon layer thereonand a binder to form a mixture, and (c) forming the mixture undertemperature and pressure conditions to a desired shape.
 2. The method ofclaim 1 wherein the carbon layer is formed on said particles bycontacting atomized alloy particles with a carbonaceous material.
 3. Themethod of claim 2 wherein the atomized alloy particles are contacted atelevated temperature in an atomizing apparatus with the carbonaceousmaterial.
 4. The method of claim 3 wherein the carbonaceous material isprovided by thermally decomposing an organic material in the atomizingapparatus.
 5. The method of claim 3 wherein the carbon layer is formedas a graphitic layer detectable by auger electron spectroscopy.
 6. Themethod of claim 2 wherein prior to step a, the atomized particles aresize classified to provide particles in a given size range.
 7. Themethod of claim 1 wherein the binder comprises a hydrocarbon polymer. 8.The method of claim 7 wherein the binder includes an olefin polymercomponent.
 9. The method of claim 4 wherein the binder comprises amixture of a first, high melt flow polyethylene and a second, stronger,moderate melt flow polyethylene.
 10. The method of claim 5 wherein thebinder comprises a 2 to 1 mixture by volume of said first and secondpolyethylenes.
 11. The method of claim 1 wherein the mixture of binderand particles is injection molded at relatively low temperaturecorresponding to the melting temperature of the lowest melting pointconstituent of the binder.
 12. A method of making a bonded isotropicpermanent magnet, comprising the steps of:a) atomizing a melt of a rareearth-transition metal alloy under conditions to form generallyspherical, rapidly solidified alloy particles having a carbon layerthereon, b) mixing a binder and the particles to form a mixture, and c)forming the mixture under temperature and pressure conditions to adesired shape.
 13. The method of claim 12 wherein the atomized alloyparticles at elevated temperature are contacted in an atomizingapparatus with a carbonaceous material therein to form said carbon layerthereon.
 14. The method of claim 13 wherein the carbonaceous material isprovided by thermally decomposing an organic material in the atomizingapparatus.
 15. The method of claim 13 wherein the carbonaceous layer isformed as a graphitic layer detectable by auger electron spectroscopy.16. The method of claim 12 wherein said particles are size classifiedafter step (a) and before step (b) by at least one of screening and airclassifying to provide a particle size fraction exhibiting desirablemagnetic properties.
 17. The method of claim 12 wherein the bindercomprises a hydrocarbon polymer.
 18. The method of claim 17 wherein thebinder comprises an olefin polymer component.
 19. The method of claim 18wherein the binder comprises a mixture of a first, high melt flowpolyethylene and a second, stronger, moderate melt flow polyethylene.20. The method of claim 19 wherein the binder comprises a 2 to 1 mixtureby volume of said first and second polyethylenes.
 21. The method ofclaim 12 wherein the mixture of binder and particles is injection moldedat relatively low temperature corresponding to the melting temperatureof the lowest melting point constituent of the binder.
 22. A method ofmaking a sintered isotropic permanent magnet, comprising the steps of:a)forming a carbon layer on rare earth-transition metal alloy particles bycontacting said alloy particles and a carbonaceous material, b) mixingthe rare earth-transition metal particles having the carbon layerthereon and a binder to form a mixture, c) forming the mixture to adesired shape body, d) removing the binder from the body, and e)sintering the body at elevated temperature.
 23. The method of claim 22wherein atomized alloy particles at an elevated particle temperature arecontacted with a carbonaceous material to form said carbon layerthereon.
 24. The method of claim 23 wherein the atomized alloy particlesare contacted at said elevated particle temperature in an atomizingapparatus with the carbonaceous material.
 25. The method of claim 24wherein the carbonaceous material is provided by thermally decomposingan organic material in the atomizing apparatus.
 26. The method of claim25 wherein the carbon layer is formed as a graphitic layer.
 27. Themethod of claim 22 wherein the binder includes an olefin polymercomponent.
 28. The method of claim 27 wherein the binder comprises amixture of a first, high melt flow polyethylene and a second, stronger,moderate melt flow polyethylene.
 29. The method of claim 28 wherein thebinder comprises a 2 to 1 mixture by volume of said first and secondpolyethylenes.
 30. A method of making a sintered isotropic permanentmagnet, comprising the steps of:a) atomizing a melt of a rareearth-transition metal alloy under conditions to form generallyspherical, rapidly solidified alloy particles having a carbon layerthereon, b) mixing a binder and the particles to form a mixture, c)forming the mixture to a desired shape body, d) removing the binder fromthe body, and e) sintering the body at elevated temperature.
 31. Themethod of claim 30 wherein atomized alloy particles at an elevatedparticle temperature are contacted in an atomizing apparatus with acarbonaceous material therein to form said carbon layer thereon.
 32. Themethod of claim 31 the carbonaceous material is provided by thermallydecomposing an organic material in the atomizing apparatus.
 33. Themethod of claim 30 wherein the carbon-bearing layer is formed as agraphitic layer.
 34. The method of claim 30 wherein the binder includesan olefin polymer component.
 35. The method of claim 34 wherein thebinder comprises a mixture of a first, high melt flow polyethylene and asecond, stronger, moderate melt flow polyethylene.
 36. The method ofclaim 35 wherein the binder comprises a 2 to 1 mixture by volume of saidfirst and second polyethylenes.